I do not think you are understanding me....Its definitely a T5
T5 indicates fixture type. The full name is a T5HO. There are other linear fixtures like a T8. T8's are not typically used anymore on cham cages. So yes you more then likely have a T5 or the other name for it T5HO fixture.
BUT the bulbs that go in them for UVB have different strengths. UNLESS YOU bought a fixture that came with a daylight bulb that you did not change out for a uvb bulb.
You have one of two things going on here and the ONLY way to know which is for you to go look at the ends of that bulb on the glass. Take a picture of what is printed on the bulb for me. If you have a daylight bulb they are getting NO uvb which is killing them. If you are using a UVB bulb at 3-4 inches above them you are over exposing them to an extremely high UVI level which is killing them.
The only way I can help you is if I know exactly what the bulb is that is in your fixture.